1. Field of the Invention
The present invention relates to an optical recording medium and a process for manufacturing the same. More particularly, the present invention concerns an organic protecting layer by which the life time of an optical recording medium, particularly a magneto-optical recording medium, is prolonged.
2. Description of the Related Art
The development and production of optical recording discs as memory devices having the ability to store a large amount of information is widespread in the communication technology field, and of these devices, research into the development of a practical magneto-optical recording disc capable of rewriting is particularly active.
A typical magneto-optical recording disc comprises a transparent plastic substrate of polycarbonate, a magneto-optical recording film of TbFeCo, a dielectric layer of ZnS between the plastic substrate and the magneto-optical recording film, and a protecting layer of ZnS over the magneto-optical recording film. Note, the magneto-optical recording film is usually protected by a dielectric layer on both sides or one side thereof, and as a result, the magneto-optical recording film has a remarkably prolonged life time in comparison with a film not having this protection, i.e., direct exposure of the magneto-optical recording film to the atmosphere. This prolonged life time is about 5 to 6 years at present.
Nevertheless, a life time of at least 10 years is required for a practical recording disc, and thus the above protection is insufficient. For example, when a magneto-optical recording disc having the typical structure mentioned above was subjected to accelerated aging at 60.degree. C. with 90% R H, pin holes appeared in the recording film after about 50 hours and the C/N (carrier to noise ratio) and BER (bit error rate) were degraded.
Further, if during storage of a magneto-optical recording disc the magneto-optical recording film thereof is in contact with the atmosphere, the magneto-optical recording film is selectively corroded by oxygen and humidity in the atmosphere, and thus the writing and reading of information become impossible.
Accordingly, the following measures are taken to improve the durability of the film: (i) the provision of a protecting layer on the surface of the magneto-optical recording film; (ii) the adhesion of another transparent plastic substrate to the surface of the magneto-optical recording film (single sided recording medium); and, (iii) the adhesion of two recording media so that the recording films face each other (double sided recording medium).
Attempts have been made to provide an inorganic layer of AlN, SiO.sub.2 or the like, as the protecting layer mentioned above, by evaporation, but the obtainable inorganic moisture sealing layer is not uniform and has an insufficient moisture sealing ability, resulting in an inevitable deterioration of the magneto-optical recording film with an elapse of time.
Attempts also have been made to form a protecting layer of a cold-cured resin, but this layer too has an insufficient moisture sealing ability, and a permeation of moisture and oxygen completed before the curing is causes a deterioration of the recording film.
Therefore, the main objects of research into an improvement of the durability of a magneto-optical recording disc are the adhesion of another plastic substrate as a protecting layer or the adhesion of two magneto-optical recording discs, as mentioned above. In the manufacture of a magneto-optical recording disc having a lamination by adhesion structure, however, the following problems arise: (i) a deterioration of the recording film by oxidation by an active ingredient such as an acidic material in the adhesive (resin); and (ii) the strain causing, for example, a tilt or warping of the recording disc and resultant cracks in the recording film, due to the shrinking of the adhesive (resin) during the curing thereof.
When a resin composition for adhesion contains a small amount of an acidic material and is in contact with a magneto-optical recording film, a rapid deterioration by oxidation of the magneto-optical recording film containing a rare earth metal such as Tb, Dy, Nd, etc., occurs. This problem is more serious when a thermosetting resin requiring a long curing time or containing a small amount of an acidic material such as phosphoric acid ester in the resin composition to improve adhesion with a metal film such as aluminum, is used.
When using a thermosetting resin requiring a long heat curing time, the transparent plastic substrate is thermally deformed (strain, etc.) and due to shrinkage of the cured resin, the laminated recording medium is tilted or warped and a recording film in contact with the cured resin is subject to a strain which may result in the appearance of cracks in the recording film.
The main object of the present invention is therefore to provide an optical recording medium which resists deterioration by humidity and heat and is stable in a humid atmosphere.